Planning fallacy is the human tendency to make optimistic predictions about how long a given task will take. This occurs because when estimating time needed we are thinking about the future instead of the past and do not accurately recall how long a task actually takes.
When people think about the past they make attributions that decrease the relevance of negative factors. An example would be the length of time it takes to travel to a particular place. Although it took an hour previously, a person is likely to decrease the importance of factors that increased the amount of time by thinking things like "Well, there was a lot of traffic that day but that won't be a factor this time" or "I was unfamiliar with the drive then, I know my way this time." Mental processes such as these lead to the planning fallacy, which occurs in our daily lives with regularity.